Such methods and treatment apparatuses are known in the state of the art. In particular in the field of ophthalmology, such methods and treatment apparatus are used for procedures with which defective vision is corrected. The cuts can be used for example to modify the cornea such that defective vision is remedied. For example, methods are known in which a volume of the cornea is isolated and removed in order to modify the curvature and thus the imaging properties of the cornea to correct defective vision.
The creation of cuts on the eye is likewise necessary in cataract surgery. Within the framework of this surgery, a cloudy crystalline lens is removed. For this removal, it is advantageous to first section the lens in the lens bag, with the result that it can be removed through a small lateral access opening created surgically.
In these fields of use, as well as in other applications, optical radiation acts inside the material, for example the tissue, which is transparent to the optical radiation. Non-linear processes are usually used which require a focussing of treatment radiation, usually pulsed laser radiation, into the material, i.e. underneath the surface of the material. The creation of a cut then occurs by shifting the position of the focus in the material. It should be understood for the purposes of this description, that the shift of the focus does not necessarily require that radiation is also emitted into the focus at this time. In particular when pulsed laser radiation is used, the focus is continuously moved and laser radiation pulses are only emitted at certain times during the focus movement. Nevertheless the corresponding lens systems or pieces of device for focus movement generally move continuously, which is why the term “focus shift” here is also understood to mean the corresponding shift of the point at which optical radiation would be focussed even if such radiation is momentarily not emitted, e.g. between two laser pulses.
The accurate focussing of the laser radiation, i.e. a geometrically strongly delimited focus, is of great importance for non-linear effects, as only then can the necessary power densities in the material be achieved. This applies both to non-linear processes in which an individual focus already results in an interaction and to processes in which several laser radiation pulses which are emitted one after the other interact in order to achieve a material-cutting effect. In this regard, approaches are also known in which laser radiation pulses are emitted at several overlapping focus spots and only the interaction of the several laser radiation pulses in the overlap area leads to material cutting.
The necessary precise focussing of the laser radiation is understandably impaired by the material through which the laser radiation is guided. Since, as already mentioned, the focus lies inside the material when cuts are to be created in the material, cuts can obviously be generated with this principle as a rule only in a direction contrary to the main direction of incidence of the laser radiation, thus from posterior to anterior when used on the eye. Otherwise, parts of the material in which material has already been cut, thus the cut has been already partially constructed, would disrupt the passage of the laser radiation and thus the desired precise focussing. In other words, areas of the cut that are deeper in relation to the direction of incidence of the optical radiation must be cut before areas of the cut that are higher can be created.
A further problem which occurs with in the creation of cuts by guiding a focus along a path is the speed of the creation of cuts. The focus is usually deflected by means of scanning device. The deceleration, re-positioning or acceleration of the scanning device can substantially prolong the creation of cuts. When used on the eye, not only is this onerous for a patient, as the surgical procedure lasts longer, the expenditure that must be met for precautions against unintended eye movements also increases with the increasing time required for the creation of cuts.
This problem is particularly great when a sectioning of transparent material is to be carried out, thus crossing cuts are required. Because of the crossover points and the fact that the cuts must be constructed in layers contrary to the direction of incidence of the radiation, the deceleration, re-positioning and acceleration of the scanning device leads to a very great prolonging of the creation of cuts.
Although approaches are known in the state of the art for carrying out the deflection movements as continuously as possible, thus for dispensing with deceleration and acceleration processes for the deflection device as far as possible (cf. DE 102008027358 A1), these approaches are limited to quite particular cut geometries and in particular are not useful for sectioning material with crossing cuts.